Simple collimation of x-rays in conjunction with tight geometries allow hand-held analyzers based on x-ray fluorescence (XRF) to achieve high-level performance on centimeter-scale targets. FIG. 1 shows a schematic of the arrangement of components of an XRF analyzer, typified by Niton XRF analyzers manufactured by Thermo Electron Corporation, and designated, generally, by numeral 10. An x-ray tube x-ray tube 100 emits a broad spectrum of x-ray emission 112 due to the acceleration of electrons (or other charged particles) 114 toward a target, referred to herein, without limitation, as anode 116. The energy spectrum of x-ray beam 112 is tailored by one or more x-ray filters 118, collimated by collimator 126 to form collimated beam 128, and directed (by pointing the instrument 10) toward a sample 120 (otherwise referred to, herein, as a “target”). Fluorescent x-rays 122 emitted by the sample are detected by detector 124.
Typical target areas interrogated by x-ray beam 112 are greater than 5 mm2, while typical target-to-anode and target-to-detector distances are less than 15 mm. In the Niton XL model XRF analyzer, the length of the x-ray tube is less than 5 cm.
The upper curve 200 of FIG. 2 shows the output intensity versus energy spectrum from a prior art gold-anode x-ray tube source operating at 50 keV. The bremsstrahlung continuum spectrum 200 is not optimum for measuring low concentration levels. The signal to noise for a given atomic element can be increased substantially by shaping the beam with filters. The lower curve 202 of FIG. 2 is an example of a filtered spectrum that is especially useful for measuring the 23.2 keV characteristic x-rays of the toxic element cadmium whose K electrons are bound with an energy of 26.7 keV. The gain in signal to noise over that obtained with an unfiltered spectrum more than a factor of 10.
X-ray focusing optics can increase the useful flux from an x-ray tube onto a target by orders of magnitude. As used herein and in any appended claims, the term “focusing optics” refers to any member of the class of devices that increase the intensity of the x-rays on a target over that which would be obtained if the optics were not used. The terms “x-ray lens” and “x-ray concentrators” are used herein as equivalent to “focusing optics,” without limitation, unless the context dictates otherwise.
Basic elements of an exemplary prior art method for focusing x-rays are described with reference to FIG. 3. The x-ray production region 301 on the anode 116 of x-ray tube 100 is the “object” (in an optical sense) of the focusing element 303 that concentrates a portion of the x-ray spectrum onto the target 120, with an illuminated area typically less than 0.1 mm2. X-ray production region 301 may sometimes be referred to as an x-ray production “point.” To achieve this concentration, the size of the electron beam spot 305 on the anode 116 is typically commensurate with the resolution on the target. It is to be understood that other polychromatic sources of x-ray radiation, such as linacs, etc., may serve as x-ray sources within the scope of the present invention. Absorber 309 absorbs x-rays that do not impinge on focusing element 303 but may otherwise impinge on the target 120.
Practical optical concentrators are generally categorized on the basis of whether they make use of total reflection or Bragg scattering. The total reflection method makes use of the fact that the index of refraction of materials is less than unity for electromagnetic waves in the x-ray energy region. The condition for total reflection from a smooth glass surface is, to good approximation, Eθ≦30, where E is the x-ray energy in keV and θ is the incident angle, in milliradians, with respect to the medium surface. For example, 30 keV x-rays are totally reflected for all incident angles less than about 1 mradian, or, at a fixed incident angle of 1 mradian, all x-rays less than about 30 keV will be totally reflected.
Bragg scattering, sometimes referred to as crystalline scattering, makes use of the fact that x-rays can be coherently scattered from an oriented crystal. The condition for Bragg scattering is that 2 d sin θ=12.4 n/E, where θ and E are, as above, the angle of incidence with respect to the planes of the crystal and the x-ray energy in keV, d is the distance between planes of the crystal (the lattice spacing) in Angstroms, and n, the order number, is an integer that is typically either 1 or 2. Example: Using a crystal with a d spacing of 2 , the first order Bragg scattering for 30 keV occurs at an angle θ=5.9°.
Both the total reflection and Bragg scattering techniques are used in energy-dispersive and angular-dispersive, laboratory x-ray spectrometers. The size, weight and power requirements of these spectrometers have hitherto been incompatible with hand held or portable XRF spectrometers that must not weigh more than a few pounds and must have a battery life of many hours. The method described here makes the x-ray optical systems useful as a concentrator for hand-held XRF systems.